This invention relates to a novel polyimide-siloxane precursor and a process for producing the same.
Polyimide resins have so far been widely used for protecting materials, insulating materials and adhesives in the field of electronic equipments or films, structural materials, etc., mainly in the aspect of heat resistance. The process of using the resins has relied, in most cases, on a process of applying a precursor prior to forming a cross-linked polymer, as it is, onto an object, followed by baking to thereby complete imidization and also effect cross-linking, and various proposals have been made for improving the above-mentioned various functions and effects after baking. Such prior art, however, cannot always be said to sufficiently satisfy current diversificated, indivisualized and sophisticated needs.
For example, polyamic acids which have so far been used for a polyimide precursor for electronic materials have been applied in the form of their solution on a substrate, followed by baking to effect imidization and curing, but at the time of their use, various problems (drawbacks) have been raised that the coating solution therefrom has a very high viscosity to make coating workability inferior; baking requires as high a temperature as 300.degree..about.400.degree. C. which often exceeds the heat-resistant temperature of substrates; adhesion of the coating solution onto silicon wafer, glass, etc. is insufficient; etc.
As to such adhesion among these problem, a number of copolymers of polyamic acid with silicon compounds have been proposed for improving the adhesion. For example, Japanese patent application laid-open Nos. Sho 57-143328/1982, Sho 58-7473/1983 and Sho 58-13631/1983 propose a technique that a polyimide precursor obtained by replacing a portion of a diamine component as raw material by a polysiloxane terminated with diamines at both the ends thereof is used to prepare a polyimide-siloxane copolymer. In this case, however, a problem has been raised that in place of improving the adhesion to a certain extent, heat resistance lowers with the increase of the siloxane content in the resulting copolymer and also the polymerization degree decreases to lower the coating-formability. Further, Japanese patent publication Nos. Sho 58-32162/1983 and Sho 58-32163/1983 disclose a process wherein a suitable carboxylic acid derivative such as tetracarboxylic acid dianhydrides is reacted with a diamine, to form a polyamidecarboxylic acid having a terminal group such as acid anhydrides, followed by reacting this polyamidecarboxylic acid with an aminosilicon compound at -20.degree. C. to +50.degree.C., to obtain a silicon-containing polyamidecarboxylic acid prepolymer (a precursor), which is not imidized or imidized (chemically cyclized) under mild conditions (low temperature, preferably 50.degree. C. or lower, particularly -20.degree. C. to +25.degree. C.) in the presence of a dehydrating agent to form an organosilicon-modified polyimide precursor, followed by heating the former unimidized precursor or the latter polyimide precursor in the form of solution in the presence or absence of a silane diol or a siloxane diol to effect completion of imidization and also cross-linking to thereby obtain a polyimide-siloxane precursor. However, this polyimide-siloxane precursor has raised various problems that it requires to bake it at a high temperature of about 200.degree. C. or higher, up to 350.degree. C. for its imidization as in the case of conventional polyimide precursor composed mainly of polyamidecarboxylic acid; if the resulting cyclized substance has a high silicon content, the coating formability is inferior, while if it has a low silicon content, adhesion onto silicon wafer, glass, etc. is inferior; and in the case where a preimidized polyimide-siloxane precursor is prepared, cyclization by low temperature treatment in the presence of a dehydrating agent requires a long time and hence this is not practical, while if cyclization is promoted by heating, the whole solution gels to lose fluidity.
In view of the above various problems of the prior art, it has been desired to develop a precursor which is soluble in a suitable solvent; has a suitable viscosity in the form of solution to afford good operability; can be baked and cured at a relatively low temperature and for a relatively short time and has a good coating-formability; and has superior adhesion onto silicon wafer, glass, etc., so that the resulting solution may be suitable to surface-protection of semiconductors, insulating film between multilayer interconnections, etc.
The object of the present invention is to provide such a soluble polyimide-siloxane precursor having overcome the above problems (in a first aspect) and a process for producing the same (in a second aspect).